EXCLUSIVE-Rights groups preparing to file lawsuit against Trump's transgender troops order, source says

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WASHINGTON, Jan 28 (Reuters) - Transgender rights advocacy groups are preparing to file a lawsuit as early as Tuesday against U.S. President Donald Trump's executive order that targeted transgender service members, in what would be the first legal challenge to a cornerstone of his conservative agenda at the Pentagon.

Trump signed an executive order on Monday that took aim at transgender troops in a personal way -- at one point saying that a man identifying as a woman was "not consistent with the humility and selflessness required of a service member."

A source familiar with the matter said GLAD Law and the National Center For Lesbian Rights (NCLR) would be filing a joint lawsuit arguing that the executive order violates the equality guarantees of the U.S. Constitution.

The executive orders signed by Trump said that expressing a "gender identity" different from an individual's sex at birth did not meet military standards.

While the order banned the use of "invented" pronouns in the military, it did not answer basic questions including whether transgender soldiers currently serving in the military would be allowed to stay and, if not, how they would be removed.